EP1343035B1 - System for low-loss splicing of optical fibers having a high concentration of fluorine to other types of optical fiber - Google Patents
System for low-loss splicing of optical fibers having a high concentration of fluorine to other types of optical fiber Download PDFInfo
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- EP1343035B1 EP1343035B1 EP20020005052 EP02005052A EP1343035B1 EP 1343035 B1 EP1343035 B1 EP 1343035B1 EP 20020005052 EP20020005052 EP 20020005052 EP 02005052 A EP02005052 A EP 02005052A EP 1343035 B1 EP1343035 B1 EP 1343035B1
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- Prior art keywords
- fiber
- holding block
- splice
- fiber holding
- thermal treatment
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/255—Splicing of light guides, e.g. by fusion or bonding
- G02B6/2551—Splicing of light guides, e.g. by fusion or bonding using thermal methods, e.g. fusion welding by arc discharge, laser beam, plasma torch
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02214—Optical fibres with cladding with or without a coating tailored to obtain the desired dispersion, e.g. dispersion shifted, dispersion flattened
- G02B6/02219—Characterised by the wavelength dispersion properties in the silica low loss window around 1550 nm, i.e. S, C, L and U bands from 1460-1675 nm
- G02B6/02252—Negative dispersion fibres at 1550 nm
- G02B6/02261—Dispersion compensating fibres, i.e. for compensating positive dispersion of other fibres
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/036—Optical fibres with cladding with or without a coating core or cladding comprising multiple layers
- G02B6/03616—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference
- G02B6/03638—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only
- G02B6/03644—Optical fibres characterised both by the number of different refractive index layers around the central core segment, i.e. around the innermost high index core layer, and their relative refractive index difference having 3 layers only arranged - + -
Definitions
- the present invention relates generally to improvements to techniques used to splice optical fiber, and more particularly to advantageous aspects of systems and methods for low-loss splicing of optical fibers having a high concentration of fluorine to other types of optical fiber.
- DCF dispersion-compensating fiber
- SSMF standard single-mode fibers
- DCF uses a small core with a high refractive index, having a mode-field diameter of approximately 5.0 ⁇ m at 1550 nm, compared with the approximately 10.5 ⁇ m mode-field diameter of SSMF at 1550 nm.
- the difference in core diameters results in significant signal loss when a fusion splicing technique is used to connect DCF to SSMF. It is possible to reduce the amount of signal loss by choosing splicing parameters that allow the core of the DCF to diffuse, thereby causing the mode-field diameter of the DCF core to taper outwards, resulting in a funneling effect.
- EP 1094346 A1 discloses a method for reducing splice loss in an optical fiber joint, in which heat is applied to the splice region to create a longitudinal diffused region comprising a length of both of the spliced fibers, wherein the amount of diffusion increases as the splice is approached along each fiber.
- a thermal treatment station comprises a chassis to which there is mounted a pair of side supports.
- a fiber holding block rests on the upper surfaces of the side supports and further includes a pair of legs that straddle the outer surfaces of the side supports.
- the fiber holding block includes a cutaway portion that exposes the splice point to a torch.
- the fiber holding block includes a second cutaway portion that allows the weight of the fiber positioned in the second cutaway portion to create a desired amount of tension at the splice point.
- there is mounted to the fiber holding block a laser that is used to create an interference pattern for precisely locating the splice point.
- One aspect of the present invention provides a technique for splicing fibers with a relatively high concentration of fluorine to other types of optical fiber.
- a suitable use for this technique is to reduce splice loss when a dispersion compensating fiber (DCF) is spliced to other types of optical fibers having a larger spotsize than the DCF, such as a standard single mode fiber (SSMF).
- DCF dispersion compensating fiber
- SSMF standard single mode fiber
- mode-field expansion of the DCF using electrical arc heating is combined with thermal-induced diffusion of the DCF dopants using a flame, furnace or other suitable heating source. This thermal-induced diffusion of optical dopants is also known as the "thermally-diffused expanded core" (TEC) technique.
- TEC thermoally-diffused expanded core
- Fig. 1A shows a cross section of a sample of a typical DCF 10
- Fig. 1B shows the refractive index (RI) profile 20 corresponding to the DCF 10 shown in Fig. 1A
- the DCF includes a core 12 surrounded by cladding that includes first, second, and third cladding regions 14, 16, and 18.
- Fig. 1A shows a core 12 surrounded by cladding that includes first, second, and third cladding regions 14, 16, and 18.
- the RI profile includes a central spike 22 corresponding to the DCF core 10, a trench 24 on either side of the spike 22 corresponding to the first cladding region 14, a ridge 26 on either side of each trench 24 corresponding to the second cladding region 16, and a flat section 28 on either side of each ridge 26 corresponding to the third cladding region 18.
- DCF is typically fabricated from a silicon dioxide (SiO 2 ) based glass.
- the desired RI profile is achieved by doping the core 12 and cladding regions 14, 16, and 18, with suitable dopants.
- the core 12 is doped with germanium (Ge)
- the first cladding region is doped with fluorine (F)
- the second cladding region is doped with germanium and fluorine (G/F)
- the third cladding region is doped with fluorine (F) at a lower concentration than the first cladding region.
- the third cladding region is not doped.
- the first cladding region 14 is doped with a relatively high concentration of fluorine dopant. Because fluorine starts to diffuse at a much lower temperature than the typical temperatures reached during fusion splicing, a significant amount of fluorine diffusion may occur during a typical fusion splicing operation. This diffusion results in a relatively high splice loss unless very short fusion times are used.
- the splice is now heated using either a flame or a furnace.
- the temperature is tuned such that fluorine diffusion starts, but no significant germanium diffusion occurs, i.e., the core expansion of the DCF is maintained.
- no significant change to the RI profile of the second fiber will occur as long as the second fiber is not as heavily doped with fluorine as the DCF. Due to the thermal treatment, the fluorine profile in the DCF is now made smooth, and the splice loss is reduced to a value below what can be obtained only using a fusion splicer.
- the above steps are now explained in greater detail with respect to the splicing together of a length of DCF with a length of SSMF.
- the splice is made while loss is monitored and the heating arc is kept on until a target splice loss value is reached, at which point the arc is turned off.
- the target splice loss value as well as the splice program parameters, such as the arc current, are determined by optimization for the actual DCF and SSMF used. Details on the implementation of the method will depend on the actual splicer used.
- the DCF is spliced to the SSMF using a relatively short fusion time, e.g., 0.3 seconds.
- a service mode is entered into in which the arc can be turned on and off manually.
- a low arc current is used, e.g., 11 mA, to ensure that the process is so slow that it is possible to observe the evaluation of splice loss manually.
- the arc is then turned off when the desired value is reached.
- Optimal splice loss after the splicing will typically be in the range of 3-6 dB.
- FIG. 2 shows a thermal treatment station 30 illustrating a further aspect of the present invention, which is used to heat a spliced optical fiber 32.
- the splice point 34 of the optical fiber 32 is positioned over the flame 36 of a gas torch 37.
- the torch's gas supply 38 is provided with a mass flow controller 40.
- a chimney 42 is located over the torch 36 to stabilize the flame during heating.
- the fiber 32 and chimney 42 are held in position by a plate 44 that includes a cutaway portion 45 for exposing the splice point 34.
- the fiber 32 is held in position on the plate 44 by first and second clamps 46 and 48 located on either side of the cutaway portion 45, and the chimney 42 is held in position on the plate 44 by an arm 50 that grips the chimney 42.
- a slight tension is maintained in the fiber 32 during the heating process by a weight that is removably attached to one end of the fiber. This tension prevents the fiber 32 from moving relative to the flame during the heating process. Care must be taken to decide the right weight to avoid stretching of the fiber when it is heated. In the present example, a weight of 0.7 g is used.
- the first clamp 46 holds the fiber 32 sufficiently loosely to allow the tension in the fiber to be controlled in this manner and functions more as a guide than a clamp. To prevent bending damage to the fiber, a curved guide is provided, upon which the weighted portion of the fiber rests during the heating process.
- the plate 44 is movable relative to the torch 36 using a translation stage 58 upon which the plate 44 is mounted.
- a position reading device 60 is provided that provides precise information as to location of the plate 44.
- the plate 44 When the spliced fiber 32 is mounted into the thermal treatment station 30, the plate 44 is positioned far above the flame. After mounting, the splice point 34 is moved into the flame using the translation stage 58. For repeatable results, the position of the translation stage 58 is monitored using the position reading device 60. Once an optimal position for the splice point 34 with respect to the flame 36 is determined, this position is used for subsequent thermal treatments.
- the torch 37 is fabricated from a quartz tube having an inner diameter of approximately 4 mm. Since the temperature necessary to diffuse the fluorine is estimated to be approximately 1200-1300° C, a gas such as propane or hydrogen without an additional oxygen supply can be used.
- the mass flow controller 40 is used to keep the gas flow at the right value. Typical flows are about 10 ml/min (for propane). Again, this value must be optimized for the particular fibers used.
- the splice loss is monitored while the splice 34 is in the flame 36.
- the translator 58 is used to remove the splice 34 from the flame 36.
- the splice 34 can now be removed from the thermal treatment station 30.
- the thermal treatment station design shown in Fig. 2 requires only 1 cm of bare fiber at the splice point 34. This is useful for compact splice protection.
- Fig. 3 shows a flowchart illustrating a method 70 embodying the above-described technique.
- the first and second fibers are spliced together using a fusion splicer.
- the fusion splicing parameters are chosen to achieve an optimal mode-field expansion of the core of the first fiber in order to reduce splice loss resulting from mismatch in core sizes of the first and second fibers.
- the spliced fiber is loaded into a thermal treatment station, such as the station illustrated in Fig. 2 .
- the splice point is positioned in the flame. This can be accomplished either by moving the splice point, moving the torch, or moving both the splice point and the torch.
- step 78 the splice loss is monitored while the splice is in the flame.
- step 80 the splice is removed from the flame when a desired minimum splice loss is achieved.
- step 82 the thermally treated fiber is removed from the thermal treatment station.
- Fig. 4 shows a graph 90 illustrating typical behavior of splice loss as a function of time of thermal treatment.
- the lowest splice loss that can be obtained for the actual DCF design is approximately 0.8 dB, when only a fusion splicer is used.
- One feature of the present technique is that it can be performed in a relative short amount of time. Using the fusion splicer to splice together the two fibers and create the desired core expansion typically requires only a few minutes. Using thermal treatment to create the desired diffusion of the fluorine dopant typically requires only 10 minutes, approximately, for reaching a minimum splice loss value.
- Fig. 5 shows a thermal treatment station 100 for use in performing the TEC technique described above.
- the thermal treatment station 100 includes, positioned on top of a chassis 101, a fiber holding block 102 having a series of clamps 104 for holding the optical fiber to be heat treated, a torch 106 for applying a flame to the fiber, and a chimney 108 for stabilizing the torch flame.
- the gas supply includes a pair of gas lines 110 for carrying, respectively, a flammable gas and oxygen. Gas flow through each line is regulated by a flow controller 112.
- each gas line includes a filter 114 and a valve 116, and both lines are fed through a gas alarm 118.
- the thermal treatment station 100 provides for translational movement, along the x-axis, ⁇ -axis, and z-axis, of the fiber held in the fiber holding block 102 relative to the torch 106.
- the fiber holding block 102 is mounted such that it may be manually pulled along the ⁇ -axis, that is, towards the operator of the station, and then pushed back into position.
- a translation stage 120 provides for precise translational movement of the torch 106 along the x, y, and z axes. The precise position of the torch 106 can be monitored by three screws protruding 122 from the translation stage 120, each screw corresponding to an axis of translational movement.
- the station 100 is provided with a lamp 124 or other device to provide suitable illumination of the splice point.
- a further embodiment of the invention includes a laser or other suitable device mounted relative to the splice point, such that is can be used to assist in the alignment of the splice with respect to the flame.
- Fig. 6A shows an exploded perspective view of the thermal treatment station 100 shown in Fig. 5 , with the gas supply elements removed for purposes of illustration.
- Fig. 6B shows an assembled perspective view of the thermal treatment station 100.
- the station 100 includes a chimney holder 130 for holding the chimney 108 in position over the flame, and a torch holder 132.
- the station 100 includes a small mounting block 134 that is used to hold a heating wire assembly that is used to insure that the torch flame has not been extinguished inadvertently.
- the heating wire assembly is illustrated in Fig. 10 , described below.
- the chimney holder 130, torch holder 132, and mounting block 134 are all mounted to the translation stage 120 so that they move as a single unit with respect to the fiber holding block 102.
- the chimney holder 130 insures that the chimney is located directly about the flame to stabilize it. When the chimney is properly positioned, a relatively short length of bare fiber is required to avoid the flame burning the fiber coating near the splice point. This is useful for situations in which the final product packaging requires very short splice protection.
- Fig. 6A further shows the side supports 136, upon which the fiber holding block 102 rests.
- the fiber holding block 102 is slidably disposed onto the top surfaces of the side supports 136 such that the fiber holding block may be moved by a station operator along the y-axis.
- the fiber holding block 102 includes a pair of legs, which closely straddle the outer surfaces of the side supports 136. This arrangement prevents the fiber holding block 102 from being slid off-axis as it is moved back and forth by the station operator. It will further be seen in Fig.
- each side support 136 includes an upwardly protruding member 138 at the rear of its upper surface that functions as a backstop to establish a working position for the fiber block.
- Fig. 6C shows a perspective view of the translation stage 120 shown in Figs. 5 , 6A and 6B .
- Fig. 6C illustrates the three axes of movement controlled by turning the screws 122. Movement along the x-axis is side to side, movement along the y-axis is in and out, and movement along the z-axis is up and down.
- Fig. 7 shows a diagram of the components of the gas and oxygen supply lines shown in Fig. 5 as a single element 110.
- the two supply lines are now separately identified as a first supply line 110a that supplies a flammable gas or mixture of gases to the torch 106 and a second supply line 110b that supplies oxygen to the torch 106.
- each supply line includes a flashback arrestor 140a and 140b, a mass flow controller 112a and 112b, a filter 114a and 114b, and a valve 116a and 116b.
- the mass flow controllers 112a and 112b are controlled separately, and precisely control the flow of gas or oxygen, respectively, at a delivery rate ranging from 0 to 20 milliliters per minute.
- the use of oxygen allows the temperature of the flame to be increased, as required.
- Fig. 8A shows a cross section
- Fig. 8B shows a front elevation view, of a modified single jet torch 106 that may suitably be used in conjunction with the thermal treatment station 100 illustrated in Fig. 5 .
- the torch 106 includes an upper conduit 150 that is connected to an oxygen supply, and a lower conduit 152 that is connected to a gas supply.
- the upper conduit 150 terminates in a jacket 154 that surrounds the terminal end of the lower conduit 152.
- the flame end of the torch 106 includes a center jet of gas supplied by the lower conduit 152 and a ring jet of oxygen surrounding the gas jet, the ring jet of oxygen supplied by the jacket 154.
- the length of the torch is 80 mm
- the length of the jacket is 55 mm
- the center-to-center distance between the conduits is 20 mm.
- the conduits have an inner diameter of 4 mm and an outer diameter of 6 mm.
- the jacket has an inner diameter of 9 mm and an outer diameter of 1 1 mm.
- Fig. 9A shows a perspective view
- Fig. 9B shows a plan view of the fiber holding block 102, without the legs 103 shown in Figs. 6A and 6B .
- the fiber holding block 102 includes a channel 160 for receiving the optical fiber to be treated.
- the first and third clamps 162 and 166 are of a type that hold the fiber in a fixed position.
- the second clamp 164 is of a type that acts as a guide but does not fix its position. The reason for this combination of clamps is to create the correct amount of tension in the fiber at the splice point 168 when the splice point is placed into the flame for thermal treatment.
- the first and second clamps 162 and 164 are positioned on either side of a first cutaway portion 169 that exposes the splice point 168.
- the weight of the fiber itself in a "weight region" 170 is sufficient to create the correct amount of tension at the splice point 168.
- the weight region 170 is a second cutaway portion of a length that is sufficient such that a length of optical fiber overhanging this region 170 will have sufficient weight to produce the desired tension at the splice point 168. It has been found that a length of approximately 200 mm for the weight region 170 creates enough tension at the splice point to insure that the fibers do not bend due to the flame. However, at the same time, the tension is significantly low so that the heated fibers do not taper, or stretch.
- Fig. 10 shows a perspective view of a heating wire assembly 180 that is used to insure that there is a flame at the torch 106.
- the heating wire assembly 180 includes a ceramic block 182 and a length of wire 184.
- the heating wire 184 is a platinum (Pt) wire having a diameter of approximately 0.3 mm.
- the wire is heated by applying a current to it.
- the heated wire turns on the flame when the thermal treatment station 100 is started and insures that the flame is on at all times when the device is running.
- the heating wire assembly 180 is useful because gas flows are often very small, and the flame can easily be extinguished inadvertently.
- Fig. 1 1A is a plan view of the fiber holding block 102 proximate to which has been positioned a visible laser source 190.
- the laser 190 can be used to insure that the splice has been properly aligned in the fiber holding block 102 along the x-axis after the spliced fiber has been moved from a fusion splicer to the thermal treatment station 100. It is desirable for the splice to be located right at the center of the torch flame.
- the visible laser source 190 is mounted to the fiber holding block 102. As illustrated in Fig. 11B , when the laser beam passes through the splice point 194 of a spliced optical fiber 192, the result is a characteristic linear interference pattern 198 appearing in conjunction with the laser beam spot 196. By referring to this linear interference pattern, the torch can be positioned with respect to the splice point by using the translation stage 120.
- Fig. 12 shows a flowchart setting forth a method 200 for using the thermal treatment station illustrated in Figs. 5 through 11 .
- step 202 the flame is turned on by opening the gas supply and by applying current to the heating wire.
- step 204 the fiber holding block is pulled out slightly towards the station operator along the ⁇ -axis to insure that the splice is not in the flame when the splice is loaded into the fiber holding block.
- step 206 a pair of fibers that has been spliced together using a fusion splicer is loaded into the fiber holding block. The position of the spliced fibers is fixed by closing the fiber holding clamps.
- step 208 the torch is aligned along its x-axis such that the splice point is in the center of the flame. As described above, this can be accomplished by using a laser and observing the resulting interference pattern.
- step 210 the torch is moved down to its lowest position along the z-axis.
- step 212 the fiber holding block is moved inwards so that the splice is now right above the center of the torch.
- step 214 the torch is moved up to its final position. The final position can be controlled by taking a position reading on the z-axis translation screw on the translation stage.
- step 216 the splice loss is continually monitored during the thermal treatment process.
- step 218 when the minimum splice loss is obtained, the splice is removed.
- the removal of the fiber is accomplished by first moving the torch down to its lowest position along its z-axis and then moving the fiber holding block along the y-axis away from the torch.
- the splice can now be removed by undoing the clamps.
- Fig. 13 shows a table 220 setting forth typical loss values at 1550 nm resulting from applying the above described techniques to a length of DCF spliced to a length of SSMF.
- the DCF used for the splices was Standard Dispersion Compensating Fiber manufactured at Lucent Technologies Denmark I/S.
- the SSMF could, for example, be Coming SMF28 fiber.
- a combination of propane-butane and oxygen were used to obtain these splice losses. Also, it is possible to obtain splices of good strength when applying the above described techniques.
- Fig. 13 shows a table 220 setting forth typical loss values at 1550 nm resulting from applying the above described techniques to a length of DCF spliced to a length of SSMF.
- the DCF used for the splices was Standard Dispersion Compensating Fiber manufactured at Lucent Technologies Denmark I/S.
- the SSMF could, for example, be Coming SMF28 fiber.
- FIG. 14 shows a table 230 setting forth splice data at 1550 nm having some of the best break loads obtained for splices made with a high-strength splicing setup and subsequently heat-treated according to the present invention. Again, a combination of propane-butane and oxygen were used.
Description
- The present invention relates generally to improvements to techniques used to splice optical fiber, and more particularly to advantageous aspects of systems and methods for low-loss splicing of optical fibers having a high concentration of fluorine to other types of optical fiber.
- A new class of optical fibers has recently been developed known as dispersion-compensating fiber (DCF), which has a steeply sloped, negative dispersion characteristic. One use for DCF is to optimize the dispersion characteristics of already existing optical fiber links fabricated from standard single-mode fibers (SSMF) for operation at a different wavelength. This technique is disclosed in United States Patent Application Serial No.
09/596,454, filed on June 19, 2000 - An important parameter for DCF is the excess loss that results when DCF is spliced to SSMF. To obtain a highly negative dispersion, DCF uses a small core with a high refractive index, having a mode-field diameter of approximately 5.0 µm at 1550 nm, compared with the approximately 10.5 µm mode-field diameter of SSMF at 1550 nm. The difference in core diameters results in significant signal loss when a fusion splicing technique is used to connect DCF to SSMF. It is possible to reduce the amount of signal loss by choosing splicing parameters that allow the core of the DCF to diffuse, thereby causing the mode-field diameter of the DCF core to taper outwards, resulting in a funneling effect. However, the high concentration of fluorine dopant in typical DCFs limits the application of this technique, because the amount and duration of the heat required to produce the funneling effect may result in an undesirable diffusion of the fluorine dopant. There is thus a need for improved techniques for splicing DCF to SSMF that reduces splice loss below current limits.
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European Patent Application Publication No. EP 1094346 A1 discloses a method for reducing splice loss in an optical fiber joint, in which heat is applied to the splice region to create a longitudinal diffused region comprising a length of both of the spliced fibers, wherein the amount of diffusion increases as the splice is approached along each fiber. - The above-described issues and others are addressed by the present invention as defined by the claims, aspects of which provide systems for thermally treating first and second optical fibers that have been spliced together at a splice point. According to a first aspect of the invention, a thermal treatment station comprises a chassis to which there is mounted a pair of side supports. A fiber holding block rests on the upper surfaces of the side supports and further includes a pair of legs that straddle the outer surfaces of the side supports. The fiber holding block includes a cutaway portion that exposes the splice point to a torch. According to another aspect of the invention, the fiber holding block includes a second cutaway portion that allows the weight of the fiber positioned in the second cutaway portion to create a desired amount of tension at the splice point. According to a further aspect of the invention, there is mounted to the fiber holding block a laser that is used to create an interference pattern for precisely locating the splice point.
- Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.
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Fig. 1A shows a cross section of a typical dispersion compensating fiber. -
Fig. 1B shows a refractive index profile of the dispersion compensating fiber shown inFig. 1A . -
Fig. 2 shows a perspective view of a thermal treatment station illustrating an aspect of the present invention. -
Fig. 3 shows a flowchart of a method according to a further aspect of the present invention. -
Fig. 4 shows a graph illustrating the relationship between thermal treatment time and splice loss. -
Fig. 5 shows a perspective view of a further embodiment of a thermal treatment station according to the present invention. -
Fig. 6A shows an exploded perspective view, andFig. 6B shows an assembled perspective view, of the thermal treatment station shown inFig. 5 , with the gas supply lines removed for purposes of illustration. -
Fig. 6C shows a perspective view of a translation stage suitable for use in the thermal treatment station shown inFig. 5 . -
Fig. 7 shows a diagram of gas and oxygen supply lines suitable for use in the thermal treatment station shown inFig. 5 . -
Fig. 8A shows a cross section, andFig. 8B shows a front elevation view, of a torch suitable for use in the thermal treatment station shown inFig. 5 . -
Fig. 9A shows a perspective view, andFig. 9B shows a plan view, of the fiber holding block used in the thermal treatment station shown inFig. 5 . -
Fig. 10 shows a perspective view of a heating wire assembly positioned proximate to a torch head according to a further aspect of the invention. -
Fig. 11A shows a plan view of the fiber holding block shown inFig. 9A with a visible laser source positioned proximate thereto in accordance with a further aspect of the invention. -
Fig. 11B shows a perspective view of the visible laser source shown inFig. 11B , and the characteristic interference pattern that results when the visible laser beam is directed through an optical fiber splice point. -
Fig. 12 shows a flowchart of a method according to a further aspect of the invention, employing the thermal treatment station illustrated inFig. 5 . -
Fig. 13 shows a table setting forth splice loss results obtained using the thermal treatment station illustrated inFig. 5 . -
Fig. 14 shows a table setting forth splice loss and break loads for high-strength splices treated using the thermal treatment station illustrated inFig. 5 . - One aspect of the present invention provides a technique for splicing fibers with a relatively high concentration of fluorine to other types of optical fiber. A suitable use for this technique is to reduce splice loss when a dispersion compensating fiber (DCF) is spliced to other types of optical fibers having a larger spotsize than the DCF, such as a standard single mode fiber (SSMF). According to one aspect of this technique, mode-field expansion of the DCF using electrical arc heating is combined with thermal-induced diffusion of the DCF dopants using a flame, furnace or other suitable heating source. This thermal-induced diffusion of optical dopants is also known as the "thermally-diffused expanded core" (TEC) technique.
- In general, DCF is difficult to splice because of its refractive index (RI) profile.
Fig. 1A shows a cross section of a sample of atypical DCF 10, andFig. 1B shows the refractive index (RI)profile 20 corresponding to theDCF 10 shown inFig. 1A . As shown inFig. 1A , the DCF includes a core 12 surrounded by cladding that includes first, second, andthird cladding regions Fig. 1B , the RI profile includes acentral spike 22 corresponding to theDCF core 10, atrench 24 on either side of thespike 22 corresponding to thefirst cladding region 14, aridge 26 on either side of eachtrench 24 corresponding to thesecond cladding region 16, and aflat section 28 on either side of eachridge 26 corresponding to thethird cladding region 18. - DCF is typically fabricated from a silicon dioxide (SiO2) based glass. The desired RI profile is achieved by doping the
core 12 andcladding regions core 12 is doped with germanium (Ge), the first cladding region is doped with fluorine (F), the second cladding region is doped with germanium and fluorine (G/F), and the third cladding region is doped with fluorine (F) at a lower concentration than the first cladding region. Alternatively, in some DCF designs, the third cladding region is not doped. - To obtain a sufficiently
deep trench 24 on either side of the core 22, thefirst cladding region 14 is doped with a relatively high concentration of fluorine dopant. Because fluorine starts to diffuse at a much lower temperature than the typical temperatures reached during fusion splicing, a significant amount of fluorine diffusion may occur during a typical fusion splicing operation. This diffusion results in a relatively high splice loss unless very short fusion times are used. - Certain optical transmission link designs require DCF to be spliced to another type of fiber, such as SSMF. In these designs, both the
core 22 anddepressed regions 24 in the DCF must commonly be altered to obtain a low-loss conversion of the DCF optical profile to a profile having a spotsize similar to the spotsize of the other fiber. Because fluorine and the core dopant, which is typically germanium, start to diffuse at different temperatures, it is possible to cause the diffusion of the core and the depressed region to occur in a two-step process. First, the DCF is spliced to the other fiber with a standard fusion splicing using a splice program that is optimized for making the right expansion of the germanium core. At the same time, fluorine, which diffuses at a lower temperature than germanium, is diffused significantly. Hence, at this point in the process, the splice loss is very high due to the diffusion of the fluorine dopant. - The splice is now heated using either a flame or a furnace. The temperature is tuned such that fluorine diffusion starts, but no significant germanium diffusion occurs, i.e., the core expansion of the DCF is maintained. At the same time, no significant change to the RI profile of the second fiber will occur as long as the second fiber is not as heavily doped with fluorine as the DCF. Due to the thermal treatment, the fluorine profile in the DCF is now made smooth, and the splice loss is reduced to a value below what can be obtained only using a fusion splicer.
- The above steps are now explained in greater detail with respect to the splicing together of a length of DCF with a length of SSMF. The splice is made while loss is monitored and the heating arc is kept on until a target splice loss value is reached, at which point the arc is turned off. The target splice loss value as well as the splice program parameters, such as the arc current, are determined by optimization for the actual DCF and SSMF used. Details on the implementation of the method will depend on the actual splicer used.
- For example, for an Ericsson FSU925, the following approach yields results with high repeatability: The DCF is spliced to the SSMF using a relatively short fusion time, e.g., 0.3 seconds. After this splice, a service mode is entered into in which the arc can be turned on and off manually. A low arc current is used, e.g., 11 mA, to ensure that the process is so slow that it is possible to observe the evaluation of splice loss manually. The arc is then turned off when the desired value is reached. Optimal splice loss after the splicing will typically be in the range of 3-6 dB.
- The splice is now ready for the thermal treatment. A furnace can be used or a torch, as described in the example below.
Fig. 2 shows athermal treatment station 30 illustrating a further aspect of the present invention, which is used to heat a splicedoptical fiber 32. Thesplice point 34 of theoptical fiber 32 is positioned over theflame 36 of agas torch 37. In order to precisely regulate thetorch 36, the torch'sgas supply 38 is provided with amass flow controller 40. Achimney 42 is located over thetorch 36 to stabilize the flame during heating. Thefiber 32 andchimney 42 are held in position by aplate 44 that includes acutaway portion 45 for exposing thesplice point 34. Specifically, thefiber 32 is held in position on theplate 44 by first andsecond clamps cutaway portion 45, and thechimney 42 is held in position on theplate 44 by anarm 50 that grips thechimney 42. - A slight tension is maintained in the
fiber 32 during the heating process by a weight that is removably attached to one end of the fiber. This tension prevents thefiber 32 from moving relative to the flame during the heating process. Care must be taken to decide the right weight to avoid stretching of the fiber when it is heated. In the present example, a weight of 0.7 g is used. Thefirst clamp 46 holds thefiber 32 sufficiently loosely to allow the tension in the fiber to be controlled in this manner and functions more as a guide than a clamp. To prevent bending damage to the fiber, a curved guide is provided, upon which the weighted portion of the fiber rests during the heating process. As described further below, theplate 44 is movable relative to thetorch 36 using atranslation stage 58 upon which theplate 44 is mounted. Aposition reading device 60 is provided that provides precise information as to location of theplate 44. - When the spliced
fiber 32 is mounted into thethermal treatment station 30, theplate 44 is positioned far above the flame. After mounting, thesplice point 34 is moved into the flame using thetranslation stage 58. For repeatable results, the position of thetranslation stage 58 is monitored using theposition reading device 60. Once an optimal position for thesplice point 34 with respect to theflame 36 is determined, this position is used for subsequent thermal treatments. - In the present embodiment, the
torch 37 is fabricated from a quartz tube having an inner diameter of approximately 4 mm. Since the temperature necessary to diffuse the fluorine is estimated to be approximately 1200-1300° C, a gas such as propane or hydrogen without an additional oxygen supply can be used. Themass flow controller 40 is used to keep the gas flow at the right value. Typical flows are about 10 ml/min (for propane). Again, this value must be optimized for the particular fibers used. - The splice loss is monitored while the
splice 34 is in theflame 36. When the minimum splice loss is reached, in approximately 10 minutes, thetranslator 58 is used to remove thesplice 34 from theflame 36. Thesplice 34 can now be removed from thethermal treatment station 30. The thermal treatment station design shown inFig. 2 requires only 1 cm of bare fiber at thesplice point 34. This is useful for compact splice protection. -
Fig. 3 shows a flowchart illustrating amethod 70 embodying the above-described technique. Instep 72, the first and second fibers are spliced together using a fusion splicer. As described above, the fusion splicing parameters are chosen to achieve an optimal mode-field expansion of the core of the first fiber in order to reduce splice loss resulting from mismatch in core sizes of the first and second fibers. Instep 74, the spliced fiber is loaded into a thermal treatment station, such as the station illustrated inFig. 2 . Instep 76, the splice point is positioned in the flame. This can be accomplished either by moving the splice point, moving the torch, or moving both the splice point and the torch. Instep 78, the splice loss is monitored while the splice is in the flame. Instep 80, the splice is removed from the flame when a desired minimum splice loss is achieved. Finally, instep 82, the thermally treated fiber is removed from the thermal treatment station. -
Fig. 4 shows agraph 90 illustrating typical behavior of splice loss as a function of time of thermal treatment. For comparison, the lowest splice loss that can be obtained for the actual DCF design is approximately 0.8 dB, when only a fusion splicer is used. One feature of the present technique is that it can be performed in a relative short amount of time. Using the fusion splicer to splice together the two fibers and create the desired core expansion typically requires only a few minutes. Using thermal treatment to create the desired diffusion of the fluorine dopant typically requires only 10 minutes, approximately, for reaching a minimum splice loss value. -
Fig. 5 shows athermal treatment station 100 for use in performing the TEC technique described above. As shown inFig. 5 , thethermal treatment station 100 includes, positioned on top of achassis 101, afiber holding block 102 having a series ofclamps 104 for holding the optical fiber to be heat treated, atorch 106 for applying a flame to the fiber, and achimney 108 for stabilizing the torch flame. The gas supply includes a pair ofgas lines 110 for carrying, respectively, a flammable gas and oxygen. Gas flow through each line is regulated by aflow controller 112. In addition, each gas line includes afilter 114 and avalve 116, and both lines are fed through agas alarm 118. As described in further detail below, thethermal treatment station 100 provides for translational movement, along the x-axis, γ-axis, and z-axis, of the fiber held in thefiber holding block 102 relative to thetorch 106. First, thefiber holding block 102 is mounted such that it may be manually pulled along the γ-axis, that is, towards the operator of the station, and then pushed back into position. Second, atranslation stage 120 provides for precise translational movement of thetorch 106 along the x, y, and z axes. The precise position of thetorch 106 can be monitored by three screws protruding 122 from thetranslation stage 120, each screw corresponding to an axis of translational movement. Thestation 100 is provided with alamp 124 or other device to provide suitable illumination of the splice point. A further embodiment of the invention includes a laser or other suitable device mounted relative to the splice point, such that is can be used to assist in the alignment of the splice with respect to the flame. -
Fig. 6A shows an exploded perspective view of thethermal treatment station 100 shown inFig. 5 , with the gas supply elements removed for purposes of illustration.Fig. 6B shows an assembled perspective view of thethermal treatment station 100. As shown inFigs. 6A and 6B , thestation 100 includes achimney holder 130 for holding thechimney 108 in position over the flame, and atorch holder 132. In addition, as shown inFig. 6A , thestation 100 includes asmall mounting block 134 that is used to hold a heating wire assembly that is used to insure that the torch flame has not been extinguished inadvertently. The heating wire assembly is illustrated inFig. 10 , described below. Thechimney holder 130,torch holder 132, and mountingblock 134 are all mounted to thetranslation stage 120 so that they move as a single unit with respect to thefiber holding block 102. - The
chimney holder 130 insures that the chimney is located directly about the flame to stabilize it. When the chimney is properly positioned, a relatively short length of bare fiber is required to avoid the flame burning the fiber coating near the splice point. This is useful for situations in which the final product packaging requires very short splice protection. -
Fig. 6A further shows the side supports 136, upon which thefiber holding block 102 rests. As mentioned above, thefiber holding block 102 is slidably disposed onto the top surfaces of the side supports 136 such that the fiber holding block may be moved by a station operator along the y-axis. It will be seen inFigs. 6A and 6B that thefiber holding block 102 includes a pair of legs, which closely straddle the outer surfaces of the side supports 136. This arrangement prevents thefiber holding block 102 from being slid off-axis as it is moved back and forth by the station operator. It will further be seen inFig. 6A that eachside support 136 includes an upwardly protrudingmember 138 at the rear of its upper surface that functions as a backstop to establish a working position for the fiber block. Thus, in positioning thefiber holding block 102 over the torch, a station operator needs only to push theblock 102 forwards until it abuts the pair of upwardly protrudingmembers 138. -
Fig. 6C shows a perspective view of thetranslation stage 120 shown inFigs. 5 ,6A and6B .Fig. 6C illustrates the three axes of movement controlled by turning thescrews 122. Movement along the x-axis is side to side, movement along the y-axis is in and out, and movement along the z-axis is up and down. -
Fig. 7 shows a diagram of the components of the gas and oxygen supply lines shown inFig. 5 as asingle element 110. For the purposes of the present discussion, the two supply lines are now separately identified as afirst supply line 110a that supplies a flammable gas or mixture of gases to thetorch 106 and asecond supply line 110b that supplies oxygen to thetorch 106. As shown inFig. 7 , each supply line includes aflashback arrestor mass flow controller filter valve mass flow controllers -
Fig. 8A shows a cross section, andFig. 8B shows a front elevation view, of a modifiedsingle jet torch 106 that may suitably be used in conjunction with thethermal treatment station 100 illustrated inFig. 5 . Thetorch 106 includes anupper conduit 150 that is connected to an oxygen supply, and alower conduit 152 that is connected to a gas supply. Theupper conduit 150 terminates in ajacket 154 that surrounds the terminal end of thelower conduit 152. Thus, as shown inFig. 8B , the flame end of thetorch 106 includes a center jet of gas supplied by thelower conduit 152 and a ring jet of oxygen surrounding the gas jet, the ring jet of oxygen supplied by thejacket 154. In the present embodiment of thetorch 106, the length of the torch is 80 mm, the length of the jacket is 55 mm, and the center-to-center distance between the conduits is 20 mm. The conduits have an inner diameter of 4 mm and an outer diameter of 6 mm. The jacket has an inner diameter of 9 mm and an outer diameter of 1 1 mm. -
Fig. 9A shows a perspective view andFig. 9B shows a plan view of thefiber holding block 102, without thelegs 103 shown inFigs. 6A and 6B . Thefiber holding block 102 includes achannel 160 for receiving the optical fiber to be treated. In addition, there are threeclamps third clamps second clamp 164 is of a type that acts as a guide but does not fix its position. The reason for this combination of clamps is to create the correct amount of tension in the fiber at thesplice point 168 when the splice point is placed into the flame for thermal treatment. As shown inFig. 9B , the first andsecond clamps first cutaway portion 169 that exposes thesplice point 168. - Because the position of the fiber is not fixed with respect to the
second clamp 164, the weight of the fiber itself in a "weight region" 170 is sufficient to create the correct amount of tension at thesplice point 168. As shown inFig. 9B , theweight region 170 is a second cutaway portion of a length that is sufficient such that a length of optical fiber overhanging thisregion 170 will have sufficient weight to produce the desired tension at thesplice point 168. It has been found that a length of approximately 200 mm for theweight region 170 creates enough tension at the splice point to insure that the fibers do not bend due to the flame. However, at the same time, the tension is significantly low so that the heated fibers do not taper, or stretch. -
Fig. 10 shows a perspective view of aheating wire assembly 180 that is used to insure that there is a flame at thetorch 106. Theheating wire assembly 180 includes aceramic block 182 and a length ofwire 184. In the present embodiment of the invention, theheating wire 184 is a platinum (Pt) wire having a diameter of approximately 0.3 mm. The wire is heated by applying a current to it. The heated wire turns on the flame when thethermal treatment station 100 is started and insures that the flame is on at all times when the device is running. Theheating wire assembly 180 is useful because gas flows are often very small, and the flame can easily be extinguished inadvertently. -
Fig. 1 1A is a plan view of thefiber holding block 102 proximate to which has been positioned avisible laser source 190. Thelaser 190 can be used to insure that the splice has been properly aligned in thefiber holding block 102 along the x-axis after the spliced fiber has been moved from a fusion splicer to thethermal treatment station 100. It is desirable for the splice to be located right at the center of the torch flame. - The
visible laser source 190 is mounted to thefiber holding block 102. As illustrated inFig. 11B , when the laser beam passes through thesplice point 194 of a splicedoptical fiber 192, the result is a characteristiclinear interference pattern 198 appearing in conjunction with thelaser beam spot 196. By referring to this linear interference pattern, the torch can be positioned with respect to the splice point by using thetranslation stage 120. -
Fig. 12 shows a flowchart setting forth amethod 200 for using the thermal treatment station illustrated inFigs. 5 through 11 . Instep 202, the flame is turned on by opening the gas supply and by applying current to the heating wire. Instep 204, the fiber holding block is pulled out slightly towards the station operator along the γ-axis to insure that the splice is not in the flame when the splice is loaded into the fiber holding block. Instep 206, a pair of fibers that has been spliced together using a fusion splicer is loaded into the fiber holding block. The position of the spliced fibers is fixed by closing the fiber holding clamps. Instep 208, the torch is aligned along its x-axis such that the splice point is in the center of the flame. As described above, this can be accomplished by using a laser and observing the resulting interference pattern. Instep 210, the torch is moved down to its lowest position along the z-axis. Instep 212, the fiber holding block is moved inwards so that the splice is now right above the center of the torch. Instep 214, the torch is moved up to its final position. The final position can be controlled by taking a position reading on the z-axis translation screw on the translation stage. Instep 216, the splice loss is continually monitored during the thermal treatment process. Instep 218, when the minimum splice loss is obtained, the splice is removed. The removal of the fiber is accomplished by first moving the torch down to its lowest position along its z-axis and then moving the fiber holding block along the y-axis away from the torch. The splice can now be removed by undoing the clamps. -
Fig. 13 shows a table 220 setting forth typical loss values at 1550 nm resulting from applying the above described techniques to a length of DCF spliced to a length of SSMF. The DCF used for the splices was Standard Dispersion Compensating Fiber manufactured at Lucent Technologies Denmark I/S. The SSMF could, for example, be Coming SMF28 fiber. A combination of propane-butane and oxygen were used to obtain these splice losses. Also, it is possible to obtain splices of good strength when applying the above described techniques.Fig. 14 shows a table 230 setting forth splice data at 1550 nm having some of the best break loads obtained for splices made with a high-strength splicing setup and subsequently heat-treated according to the present invention. Again, a combination of propane-butane and oxygen were used. - While the foregoing description includes details which will enable those skilled in the art to practice the invention, it should be recognized that the description is illustrative in nature and that many modifications and variations thereof will be apparent to those skilled in the art having the benefit of these teachings. It is accordingly intended that the invention herein be defined solely by the claims appended hereto and that the claims be interpreted as broadly as permitted by the prior art.
Claims (4)
- A thermal treatment station (100), characterized by:a chassis (101);a fiber holding block (102) for holding a pair of optical fibers that have been spliced together at a splice point (168), the fiber holding block including a cutaway portion (169) exposing the splice point;a torch (106),first and second side supports (136) mounted to the chassis (101) with the torch therebetween;wherein the fiber holding block (102) rests on the upper surfaces of said pair of side supports (136) mounted to the chassis (101), the fiber holding block (102) including a pair of legs (102, 103) that straddle the side supports (136) such that the inner surfaces of the legs (102, 103) abut the outer surfaces of the side supports (136), such that the fiber holding block (102) can be moved back and forth along a path defined by the side supports (136), the fiber holding block (102) being movable between a first position for loading the spliced fibers into the fiber holding block (102) and a second position in which the splice point (168) is positioned above the torch flame (106).
- The thermal treatment station (100) of claim 1, wherein the upper surface of each side support (136) includes an upwardly protruding member (138), the upwardly protruding members (138) defining the second position of the fiber holding block (102).
- The thermal treatment station (100) of claim 1, wherein the fiber holding block (102) includes:a first clamp (162) and a second clamp (164) positioned on the fiber holding block (102) on either side of the first cutaway portion (169), the first clamp (162) holding the pair of spliced optical fibers in position, and the second clamp (164) permitting movement of optical fiber therethrough, the second clamp (164) functioning as a guide,the fiber holding block (102) including a second cutaway region (170) positioned such that the second clamp (164) lies between the first cutaway region (169) and the second cutaway region (170), the second cutaway region (170) being of sufficient length such that when a pair of spliced optical fibers is loaded into the fiber holding block with a portion of optical fiber positioned over the second cutaway region (170), the portion of optical fiber over the second cutaway region has a weight sufficient to result in a desired level of tension at the splice point (168).
- The thermal treatment station (100) of claim 1, further including:a laser source (190) mounted to the fiber holding block (102), the laser source (190) generating a laser beam that passes through the spliced optical fiber (192), perpendicular to the fiber's optical axis, the laser spot being a visible spot (196) indicating a desired location for the fiber's splice point (194),the laser beam creating a characteristic interference pattern (198) when the beam traverses the splice interface, the interference pattern indicating the axial position of the fiber splice point (194),such that a desired axial position of the splice point (194) in the thermal treatment station (100) can be achieved by axially moving the splice point (194) while referring to the relative positions of the interference pattern (198) and the laser beam spot (196).
Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
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EP20020005052 EP1343035B1 (en) | 2002-03-06 | 2002-03-06 | System for low-loss splicing of optical fibers having a high concentration of fluorine to other types of optical fiber |
DE60229492T DE60229492D1 (en) | 2002-03-06 | 2002-03-06 | Apparatus for low attenuation optical fiber splicing with high concentration of fluorine with other types of optical fibers |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
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EP20020005052 EP1343035B1 (en) | 2002-03-06 | 2002-03-06 | System for low-loss splicing of optical fibers having a high concentration of fluorine to other types of optical fiber |
Publications (2)
Publication Number | Publication Date |
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EP1343035A1 EP1343035A1 (en) | 2003-09-10 |
EP1343035B1 true EP1343035B1 (en) | 2008-10-22 |
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EP20020005052 Expired - Lifetime EP1343035B1 (en) | 2002-03-06 | 2002-03-06 | System for low-loss splicing of optical fibers having a high concentration of fluorine to other types of optical fiber |
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CH638622A5 (en) * | 1979-09-14 | 1983-09-30 | Cabloptic Sa | METHOD AND DEVICE FOR WELDING OPTICAL FIBERS. |
JPS5778512A (en) * | 1980-11-05 | 1982-05-17 | Nippon Telegr & Teleph Corp <Ntt> | Connecting method of optical fiber |
JPS59152412A (en) * | 1983-02-18 | 1984-08-31 | Nec Corp | Connecting method of polarization plane maintaining optical fiber |
US4958905A (en) * | 1989-06-19 | 1990-09-25 | Tynes Arthur R | Method and apparatus for forming high strength splices in optical fibers |
SE511966C2 (en) * | 1997-06-09 | 1999-12-20 | Ericsson Telefon Ab L M | Method and apparatus for jointing the ends of two optical fibers of different type with each other |
KR100288445B1 (en) * | 1997-12-30 | 2001-05-02 | 윤종용 | Method for coupling two optical fibers one of which has cladding diameter different from the other |
IES990889A2 (en) * | 1999-10-22 | 2001-05-02 | Viveen Ltd | Jointed optical fibers |
-
2002
- 2002-03-06 EP EP20020005052 patent/EP1343035B1/en not_active Expired - Lifetime
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